Homeless people have a 3.5-fold higher prevalence of cigarette smoking in comparison to non-homeless people, contributing to 2-fold higher rates of lung cancer and 3- to 5-fold higher rates of tobacco-attributable death. Homeless smokers want to quit, but studies have not yet uncovered the optimal approach to help them do so. In a recently published 8-week pilot randomized controlled trial (RCT) at Boston Health Care for the Homeless Program (BHCHP), we found that financial incentives for smoking abstinence were associated with 7-fold higher odds of brief smoking abstinence in comparison to a non-incentivized control condition. These results suggest that financial incentives are a promising approach for reducing smoking in this vulnerable population, although further investigation in a larger sample is needed to improve the duration of on-treatment abstinence, assess post-treatment effects, and better understand mechanisms of action and contextual factors that may influence treatment response. To address these gaps, we will conduct a community-based RCT of financial incentives for smoking abstinence among adult smokers at BHCHP. We will recruit 400 participants from 3 BHCHP sites and randomize them to receive guideline-concordant standard care with (N=200) or without (N=200) financial incentives for smoking abstinence. Standard care will consist of 8 weeks of combination nicotine replacement therapy (NRT) and 5 sessions with a tobacco coach who will provide behavioral counseling, distribute NRT, and coordinate referrals to existing tobacco treatment resources. Incentive arm participants will additionally receive a 24-week schedule of 20 debit card payments contingent on short-term abstinence based on exhaled carbon monoxide levels, augmented with 16 interspersed payments contingent on longer-term abstinence based on urine anabasine levels. We will use an embedded-experiment mixed methods design, where qualitative (?qual?) data collection is embedded within a larger quantitative (?QUAN?) RCT with the following aims: Aim 1. (QUAN) To determine the effect of the financial incentives intervention on anabasine-verified 7-day smoking abstinence at A) the end of treatment (24 weeks) and B) 24 weeks after treatment (48 weeks). We hypothesize that incentive arm participants will have greater abstinence at both timepoints. Aim 2. (qual) To assess why, how, and under what circumstances homeless smokers A) achieve abstinence in response to financial incentives and B) maintain abstinence after incentives are stopped. To accomplish this aim, we will interview selected participants at 24 and 48 weeks, probing cognitive, procedural, and contextual dimensions of their response to financial incentives to generate hypotheses about mechanisms for on-treatment and post-treatment effects and to inform future work in this area. Our findings could have important tobacco treatment implications for the 934,000 people served annually in US Health Care for the Homeless programs, advancing NCI?s objective of reducing smoking in vulnerable populations.